The present disclosure sets out an approach for separating reflected signals from refracted signals in well borehole electromagnetic tools. A typical tool used in these circumstances can have a frequency as high as several Ghz down to the Mhz range, and even lower in frequency. Generally, they transmit an EM field into formations adjacent to a well borehole utilizing a magnetic dipole antenna either arranged parallel to the axis of the well or horizontally positioned transverse to the axis or a combination of antennas or axes. The normal circumstances in which this device is used involves transmission from a logging tool sonde through a mud cake or invaded zone. The problem referred to is found in all formations including those where a mud cake is formed and also those where drilling fluid invasion into the adjacent formations does occur. The EM field directed from a transmitter antenna in a logging sonde must extend from the antenna into the formation. This requires EM propagation traversing the interface between the borehole and the adjacent formation. There is at least a first interface. This interface is defined by the drilling fluid and the adjacent formation. In some instances, there will be a distinct mud cake and region adjacent to the borehole where the filtrate from the drilling mud has penetrated. Sharply or poorly defined regions occur and hence, there may be multiple concentric interfaces. This becomes important as the EM frequency increases to the Ghz range. It also can be a substantial problem for lower frequencies for reasons described below.
In general, the EM radiation travels through the surrounding materials as a reflected and a refracted wave. In general terms, the mode of EM radiation and transmission is one of two types of signals, or both types mixed together. In one type, a refracted wave travels through the immediate or adjacent medium and is incident at the interface at the critical angle. The wave will be totally internally reflected and then travel along the interface between the two media (recall that this is a surrounding cylinder) wherein the travel velocity is defined by physical properties of the respective media. Wave energy scattering occurs at the media interface back into the first medium.
Another type of wave is obtained from pole contribution and thus is the remaining energy in the emitted EM wave which did not pass through the interface and is not incident at the critical angle. This wave is reflected at the interface, and is described hereinafter as the reflected wave. At various locations within the first medium, the total EM signal is a combination of the refracted and reflected waves.
There is the possibility dependent on the physical dimensions of the borehole and the wave length of the associated EM radiation that the reflected wave is an evanescent wave which cannot propagate out of the first medium. If the first medium is substantially thick (in multiples of one wave length), the EM radiation may be attenuated completely before the wave reaches the surrounding cylindrical interface. In that instance, there will be no refracted wave. It is difficult to know in advance whether there will be both reflected and refracted waves. Thus, one must assume that both waves exist within the first medium. This inevitably suggests all data has error. The error is trivial if one of the two waves is quite small; since relative size cannot be known in advance, one must presume that both waves are substantial and that the interference between the reflected and refracted waves is substantial. Accordingly, the interference may well cause substantial error in measurements of electrical properties otherwise obtained by the EM wave propagated in the well borehole.
The present disclosure sets forth an approach enabling the reflected and refracted waves to be measured separately. By use of this, the electrical properties of the media can be measured and the measurements can be separated so that the responses to reflected and refracted waves are both obtained. In part, this is accomplished by utilization of a polarized EM transmission into the formations. Circular polarization is preferred, and obtains a polarized response. Assume that the polarization is in the clockwise direction. On transmission, any reflections provide a reflected wave with a counterclockwise rotation. It is possible to distinguish this kind of received reflected signal. By contrast, refraction does not involve the image reversal of reflection and hence the refracted signal will carry with it the circulate polarization in the clockwise direction. This can be accomplished by simply injecting a signal from a magnetic dipole along the radial axis of the borehole. There are well known spiral or helical antenna assemblies available which will impart the desired circular polarization. Obviously, the polarization can be counterclockwise also.
Another important feature of the present apparatus is the use of an antenna which is enclosed within a material providing a dielectric constant substantially in excess of one. For instance, when operating in the gigahertz range, one wave length is quite short and it is easy to position a gigahertz range antenna within a borehole. However, at lower frequencies, perhaps in the range of 20 to 100 Mhz, another problem is encountered. One wave length is quite long and certain practical considerations come into play regarding positioning such an antenna in a typical borehole. The free space wave length at 30 Mhz is about 30 meters. It is difficult, and practically impossible to design and position a spiral or helical antenna for a tool fitting in a typical borehole from typically having a diameter of up to about eight or nine inches. However, scaling down of the antenna can be obtained by surrounding the antenna with a high dielectric constant material. For instance, a ceramic known as PZT4 can be used to provide a surrounding volume for an antenna where the dielectric constant is 1300. In that event, a helical antenna can be provided with a diameter of about 3.4 inches. A pad or skid mounted antenna thus would fit in a cylinder of about 3.4 inches and have a length of about 3 inches and would output a circular polarized field. The antenna can be multiplexed or alternately a receiver antenna can be affixed on another pad. Dependent on the direction of winding of the helix, a refracted or reflected signal can be obtained by the receiver. Separation of reflected and refracted signals particularly enhances the dielectric response of tools typically operating in the range of about 10 to about 200 Mhz. This is particularly helpful in sorting out the dimensions of the invaded zone concentric about the borehole.
By contrast, when operating at UHF, the signal is typically impacted by the mud cake. By utilizing circular polarized antennas to transmit and receive, the mud cake impact for dielectric measuring tools operating in the gigahertz range can be markedly improved. Going to the very lowest frequencies, typically induction tools which operate at 20 Khz, circular polarization of the transmitted signal can be utilized to inject the radiation into the formation at a controlled depth beyond the invaded zone so that measurements can be obtained both from the invaded regions of the formation and the univaded regions. Separate measurements can be obtained in light of the fact that the reflected and refracted signals can be sorted out. In summary, circular polarization assists markedly in sorting out reflected and refracted waves and thereby permits obtaining more accurate data.